To protect an electric motor or other electrical device from damage due to an undesirable operating condition, such as an overload, under load, etc. industrial control systems can employ, as a standard power distribution method, a method of combining a relay, such as an overload relay, which is typically in the form of a thermal overload relay or an electronic overload relay, along with an electromagnetic contactor connected to a power circuit for the electric motor. In an overload operation, the electromagnetic contactor is allowed to cut off current to stop the electric motor.
Presently available thermal overload relays utilize heater and detector elements suitable for measuring only small amperage increments per each heater and detector element. Thermal overload relays typically have a small current adjustment range of 1.5:1, meaning the maximum setting is 1.5 times the lower setting. However, there are a wide variety of industrial control systems encompassing numerous current ranges that an overload relay may have to accommodate. This requires numerous sizes to be available in order to practically address common loads. For example, a typical IEC style contactor frame size is 45 mm wide and contactors switching up to 22 A are commonly manufactured in this single frame size. For this same 45 mm frame size, over 15 different thermal overload sizes are required (e.g., 0.1-0.16 A, 0.16-0.25 A, etc up to 16-22 A) to accommodate motor protective loads up to 22 A. The sheer number of thermal overload combinations is costly to inventory and can result in incorrectly ordered and/or incorrectly sized overloads being applied.
Compared to thermal overloads, electronic overloads are capable of measuring wider current ranges by utilizing current transformers. However, current transformers are subject to saturation, therefore accuracy degrades as the magnetics of the transformer staturate with increased current. This effectively limits the applicable current ranges. The current state of the art adjustment range of presently available electronic overload relays is typically limited to approximately 3.2 to 1, meaning the maximum setting is 3.2 times the lower setting. However, this still requires numerous overload sizes to be available to address the loads covered by a typical IEC 45 mm frame size contactor. In this frame size, up to 22 A is typically switched, yet over 5 different overload sizes can still be required (e.g., 0.1-0.32 A, 0.32-1.0 A, 1.0-2.9 A, 1.6-5.0 A, 3.7-12 A). Again, the sheer number of overload combinations is costly to inventory and can result in incorrectly ordered and/or incorrectly sized overloads being applied.
Electronic overloads require power for their circuitry, which poses certain challenges, as the readily available line voltage being switched is typically far in excess of the electronic overload power supply requirements (e.g. 480 VAC line voltage vs. 24 VAC electronic overload power required). With traditional electronic overloads, this necessitates the use of an external power supply. Certain models, such as Sprecher and Shuh CEP7, induce their power from the conductor being monitored using current transformers. However, this technique has limitations, as the current transformers are also used for measurement and subject to limited current measurement range.
U.S. Pat. No. 5,715,129 (“Innes”), issued Feb. 3, 1998, teaches an electronic overload relay having a power supply in series with the normally closed contact of the overload relay. The power supply is an integral element of the electronic overload relay in Innes. The relay is connectable to an electromagnetic contactor in keeping with conventions of thermal overload relays wherein the contactor coil is connected in series with the normally closed contact of the relay, and therefore also in series with the power supply to provide power for the overload relay when power is supplied to the contactor coil. A processor in the electronic overload relay is instructed to assume a sleep (low power consumption) mode during the closing of the contactor. A semiconductor switch in the power supply is operated by the processor in low voltage coil applications to directly connect the coil of the contactor in shunt of the power supply for the relay while the contactor closes. However, while providing a technique to power the electronic overload circuitry, the device in Innes is dependent on contactor coil voltage being in a suitable range for direct input to the electronics circuitry (e.g., 24VAC). In practice, contactors are often controlled through a push button or actuated using line voltages through the contactor coil. In these instances, utilizing coil voltages to power the contactor would not be feasible due to high line voltages (e.g. 480VAC) incompatible with the device.
U.S. Pat. No. 5,589,809 (“Kogawa et al.”), issued Dec. 31, 1996, relates to an adjusting dial of a thermal overload relay for adjusting a working current of the thermal overload relay, and, more specifically, to a structure of the relay which can prevent an adjusting dial previously set from being mis-readjusted. However, Kogawa et al. still requires an initial manual setting of the thermal overload for the proper load rating, which is a labor intensive process and potentially subject to error.
Both thermal and electronic overloads require field calibration in order to establish the set-point of the normal full load amperage of the load monitored. Field calibration is a manual task, and as such, can be expensive and prone to human error. As a result, equipment may not be properly protected, nuisance trips may result, and life safety issues may arise should an overload be improperly sized or adjusted.